To extend the reach of crane booms having telescopic boom sections, a side stowable jib or boom extension may be provided for connection to the nose assembly of the fly section or next adjacent boom section, as disclosed in U.S. Pat. Nos. 3,785,505, dated Jan. 15, 1974, and 4,483,447, dated Nov. 20, 1984. The entire contents of these patents are hereby incorporated by reference.
When moving the boom extension from a storage position on the side of the telescopic boom to a use position wherein the boom extension extends outwardly in alignment with the longitudinal axis of the boom, the boom extension is pivotally connected to one side of the nose assembly of the boom section and then swung around and connected to the other side of the nose assembly. The connections are made by a plurality of pins extending through aligned holes provided in the cooperating end portions of the boom extension and nose assembly of the fly section.
On relatively small cranes, holes on the boom extension can be aligned with holes on the boom by manually pulling one end of the boom extension away from the boom and pivoting the boom extension on an intermediate support until the openings on the extension come into alignment with the openings on the boom nose. Cranes such as the one disclosed in U.S. Pat. No. 3,785,505, for example, include a roller on the boom extension that is rolled up a short ramp to carry the boom extension onto a boom extension support on the boom. While this arrangement performs satisfactorily, if an operator inadvertently removes the safety retaining pin from the intermediate support, pulling the rear portion of the boom extension away from the boom could result in the entire boom extension detaching from the boom and severely injuring the operator. Additional safety devices therefore must generally be provided to minimize this problem.
Another approach to boom extension deployment has been developed for relatively small cranes. In such cranes, the rear portion of the boom extension slides toward and away from the boom on a slider mounted on a rail. The boom extension is retained on the slider by pins projecting in the direction of the front end of the boom. Therefore, even when the slider is in the extended position, the boom extension will not detach from the slider until it is moved in the direction toward the outward end of the boom, and the extension will not detach from the boom even if an operator accidentally removes the safety pin from the intermediate support before moving the rear portion of the boom. Even with relatively small cranes, however, it is difficult for an operator to move the heavy boom extension as required to ensure that the front end of the boom extension is aligned with openings on the boom. This arrangement has therefore not been widely adopted on small cranes and is impracticable for larger cranes having heavier boom extensions.
It would therefore be desirable to provide a boom extension that is easily shiftable from a storage position to a position wherein the boom extension can be connected to a boom regardless of the size of the boom extension and to provide a boom extension that will not readily detach from a boom.